Mr. Darwin on the Ancient Glaciers of Caernarvonshire. 181 



or four linear ridges evidently mark the principal stages in the 

 retreat of the glacier ; the outer one is the longest, and di- 

 verges most from the great wall of rock at the south end of 

 the lake. The inner lines distinctly define the boundary of 

 the glacier during the last stage of its existence. At this pe- 

 riod a small and distinct glacier descended from a narrow but 

 lofty gorge on the north-western end of the lake ; and here 

 remnants of a terminal moraine may be traced in the little 

 mounds, forming a broken semicircle round a rushy plain, 

 scarcely more than a hundred yards in diameter. The rocks 

 are smoothed, mammillated and scored, all round the lake, 

 and at some little depth beneath the surface of the water, as I 

 could both see and feel. Similar marks occur at great heights 

 on all sides, far above the limits of the moraines just described, 

 and were produced at the time when the ice poured in a vast 

 stream over the rocky barrier bounding the northern end of 

 the amphitheatre of Lake Idwell. 



I may here mention, that about eighty yards west of the spot 

 where the river escapes from the lake, through a low mound 

 of detritus," probably once a terminal moraine, there is an ex- 

 ample of a boulder broken, as described by Charpentier and 

 Agassiz, into pieces, from falling through a crevice in the ice. 

 The boulder now consists of four great tabular masses, two of 

 which rest on their edges, and two have partly fallen over 

 against a neighbouring boulder. From the distance, though 

 small in itself at which the four pieces are separated from each 

 other, they must have been pitched into their present position 

 with great force ; and as the two upright thin tabular pieces 

 are placed transversely to the gentle slope on which they 

 stand, it is scarcely possible to conceive that they could have 

 been rolled down from the mountain behind them ; one is led, 

 therefore, to conclude that they were dropped nearly vertically 

 from a height into their present places. 



The rocky and steep barrier over which the ice from the 

 amphitheatre of Lake Idwell flowed into the valley of Nant- 

 Francon, presents from its summit to its very foot (between 

 400 and 500 feet) the most striking examples of boss or dome- 

 formed rocks; so much so, that they might have served as 

 models for some of the plates in Agassiz's work on Glaciers. 

 When two of the bosses stand near and are separated only by 

 a little gorge, their steep rounded sides are generally distinctly 

 scored with lines, slightly dipping towards the great valley in 

 front. The summit of the bosses is comparatively seldom 

 scored ; but on one close to the bridge over the river Ogwyn, 

 I remarked some singular zigzag scores. At this spot the 

 cleavage of the slate is highly inclined, and owing apparently 



